1. Field of the Invention
The invention relates to a control system for induction motors and to a method for controlling torque using sensorless direct torque and flux regulation.
2. Background Art
Attempts have been made in the design of controls for induction motors to use speed and flux observers in a system that lacks rotational transducers. Such sensorless control methods make it possible to achieve high dynamic performance of induction motors while achieving low manufacturing costs, high reliability, robustness, and ease of maintenance. For example, a high gain speed observer for use in an induction motor torque control is described in a paper by Khalil et al, entitled xe2x80x9cA Torque Controller for Induction Motors Without Rotor Position Sensorxe2x80x9d, International Conference on Electric Machines, Virgo, Spain, 1996. A paper written by Yoo and Ha, entitled xe2x80x9cA Polar Coordinate-Oriented Method of Identifying Rotor Flux and Speed of Induction Motors without Rotational Transducersxe2x80x9d, IEEE Transactions, Volume 4, No. 3, May 1996, describes a design that uses a separate high gain speed observer with a separate robust rotor flux observer for each of two operating modes, based on the flux natural dynamics. The speed observer and the flux observer for each mode of operation of the motor complement each other in the different modes of operation of the motor.
U.S. Pat. No. 6,316,904, issued Nov. 13, 2001, entitled xe2x80x9cSpeed and Rotor Time Constant Estimation for Torque Control of an Induction Motorxe2x80x9d, filed by Zaremba and Semenov, describes a method for estimating rotor resistance using pseudo current and voltage signals. That patent is assigned to the assignee of the present invention.
U.S. Pat. No. 6,433,506, issued Aug. 13, 2002, entitled xe2x80x9cSensorless Control System For Induction Motor Employing Direct Torque And Flux Regulationxe2x80x9d, filed by Pavlov and Zaremba, discloses a dynamic model for implementing the motor control using stator voltage as input and separately tracks torque and flux.
Kubota and Matsuse, in their paper entitled xe2x80x9cSpeed Sensorless Field-Oriented Control of Induction Motor with Rotor Resistance Adaptationxe2x80x9d, IEEE Transactions, Volume 30, No. 5, September/October 1994, describe a method for estimating speed and flux using an adaptive control technique.
A paper by Hori et al, entitled xe2x80x9cA Novel Induction Machine Flux Observer and Its Application to a High Performance AC Drive Systemxe2x80x9d, IFAC 10th Triennial World Congress, Munich, 1987, describes various design methods for estimating rotor flux in an induction motor controller. The method uses a flux observer that makes it possible to use flux feedback vector control, rather than a so-called slip frequency control, based on a simple control algorithm.
A flux observer utilizing flux dynamics having the ability to track position and velocity error and torque and flux error is described by Sun and Mills in a paper entitled xe2x80x9cAC Induction Motor Control Using an Advanced Flux Observer Designxe2x80x9d, Proceedings of the American Control Conference, Chicago, Ill., June 2000.
Sensorless control of an induction motor using the method of the invention includes adaptive observers of speed and flux. The induction motor may be used in the hybrid powertrain for an automotive vehicle wherein the induction motor complements an internal combustion engine to establish torque flow to vehicle traction wheels.
The observers are based on a special form of an induction motor model in which stator current dynamics are separated from the unobservable rotor flux. The model is obtained by assuming that rotor speed is a slowly changing variable relative to the flux and current signals.
Using this model with a separated stator current as a reference, a rotor speed identification scheme is designed in the frame of a model reference adaptive system.
In an alternative design, a speed observer is calibrated using a higher order tuning system, thereby improving the tracking of speed transients, although it may be more susceptible to current noise. In direct torque and flux control systems, and also in sensorless strategies with speed estimation, a rotor flux observer is used. Unlike conventional rotor flux observers, which are based on the integration of the rotor flux equation and which is sensitive to system errors, the design of the present invention includes stator current dynamics that are separated from the rotor flux. This allows a definition of rotor flux using an algebraic equation. This equation, which is used to construct the rotor flux observer, does not involve integration. This then greatly reduces the sensitivity of flux estimates to speed systematic error.
In practicing the invention, the method includes measuring current, filtering the current, calculating the voltage, filtering the voltage, estimating stator current and current error as a function of time, estimating and adjusting the speed, estimating the flux and calculating the rotor flux angle, which is used by a torque and flux regulator to calculate rotor torque.
The flux observer of the invention is based on an algebraic relation between measured and filtered voltage and current signals of an induction motor. Unlike known rotor flux observers, the flux observer of the invention does not integrate the rotor speed estimate. It, therefore, works better in the low rotor speed region where accurate speed estimates are difficult.
The speed observer of the invention has improved robustness, compared to indirect field orientation (IFO) systems, with respect to variations in rotor resistance. Such resistance variations can occur as the operating temperature changes.